The immune response is conceptually divided into innate immunity and adaptive immunity. Innate immunity is believed to involve recognition of pathogen-associated molecular patterns (PAMPs) shared in common by certain classes of molecules expressed by infectious microorganisms or foreign macromolecules. PAMPs are believed to be recognized by pattern recognition receptors (PRRs) on certain immune cells.
Toll-like receptors (TLRs) are a family of highly conserved polypeptides that play a critical role in innate immunity in mammals. Currently ten family members, designated TLR1-TLR10, have been identified. The cytoplasmic domains of the various TLRs are characterized by a Toll-interleukin 1 (IL-1) receptor (TIR) domain. Medzhitov R et al. (1998) Mol Cell 2:253-8. Recognition of microbial invasion by TLRs triggers activation of a signaling cascade that is evolutionarily conserved in Drosophila and mammals. The TIR domain-containing adapter protein MyD88 has been reported to associate with TLRs and to recruit IL-1 receptor-associated kinase (IRAK) and tumor necrosis factor (TNF) receptor-associated factor 6 (TRAF6) to the TLRs. The MyD88-dependent signaling pathway is believed to lead to activation of NF-kB transcription factors and c-Jun NH2 terminal kinase (Jnk) mitogen-activated protein kinases (MAPKs), critical steps in immune activation and production of inflammatory cytokines. For a review, see Aderem A et al. (2000) Nature 406:782-87.
While a number of specific TLR ligands have been reported, ligands for some TLRs remain to be identified. Ligands for TLR2 include peptidoglycan and lipopeptides. Yoshimura A et al. (1999) J Immunol 163:1-5; Yoshimura A et al. (1999) J Immunol 163:1-5; Aliprantis A O et al. (1999) Science 285:736-9. Viral-derived double-stranded RNA (dsRNA) and poly I:C, a synthetic analog of dsRNA, have been reported to be ligands of TLR3. Alexopoulou L et al. (2001) Nature 413:732-8. Lipopolysaccharide (LPS) is a ligand for TLR4. Poltorak A et al. (1998) Science 282:2085-8; Hoshino K et al. (1999) J Immunol 162:3749-52. Bacterial flagellin is a ligand for TLR5. Hayashi F et al. (2001) Nature 410:1099-1103. Peptidoglycan has been reported to be a ligand not only for TLR2 but also for TLR6. Ozinsky A et al. (2000) Proc Natl Acad Sci USA 97:13766-71; Takeuchi O et al. (2001) Int Immunol 13:933-40. Bacterial DNA (CpG DNA) has been reported to be a TLR9 ligand. Hemmi H et al. (2000) Nature 408:740-5; Bauer S et al. (2001) Proc Natl Acad Sci USA 98, 9237-42. The TLR ligands listed above all include natural ligands, i.e., TLR ligands found in nature as molecules expressed by infectious microorganisms.
The natural ligands for TLR1, TLR7, TLR8 and TLR10 are not known, although recently certain low molecular weight synthetic compounds, the imidazoquinolones imiquimod (R-837) and resiquimod (R-848), were reported to be ligands of TLR7. Hemmi H et al. (2002) Nat Immunol 3:196-200.